THE FLY 77 



group protozoa, or primitive animals and it is especially with 

 these that we have to deal in this chapter. 



The greatest difficulty that a parasite has to encounter is 

 how to find a way to pass from one incUvidual to another. After 

 it has wrought its fatal work upon the body of one individual, 

 it must be able to reach a new host in order that it may not 

 altogether perish with the host it has killed. There are two 

 principal ways in which this transfer from one individual to 

 another takes place. Either the parasites or their eggs are 

 extruded from the body of the first host and arc taken by a 

 healthy individual with its ordinary food or water, or else the 

 parasites are actually conveyed from one individual to another 

 by means of so-called intermediary hosts. These intermecHary 

 hosts are sometimes animals that devour the carcass of an 

 animal dead of a disease and are in turn eaten (with the dis- 

 ease germs) bjr a healthy individual belonging to the same 

 species as that which died, or else they are animals that suck 

 the blood of other animals, and in passing from individual to 

 individual infect, of course unwittingly, the healthy with 

 parasites from the diseased. This role of intermediary host 

 is played by the group of flies (Dip'tera ') more than by any 

 other. Consequently the Diptera are a great menace to man 

 as conveyers of many of the most dreaded diseases to which 

 he is subjected. 



The Diptera are fitted to play the role in nature of disease- 

 bearers because of their hfe history, instincts, and structure. 

 For the most part they are lovers of heat, so that they are most 

 abundant in the tropics. It is the Diptera and the cUseases 

 they bear, not the heat, that make the tropics such an un- 

 healthy or even fatal abiding-place for man and for many of his 



' dis, twice; pteron, wing. 



